The 3rd Generation Partnership Project (3GPP) Long Term Evolution-Advanced (LTE-A), which is the next-generation mobile communication system, is under standardization, and its standardization is expected to be completed by 2011. To provide better performance than the existing high-speed packet-based communication, several schemes are under discussion in LTE-A. Among them, typical schemes include a Carrier Aggregation (CA) technology of transmitting data using several frequency bands altogether, and a Minimization of Drive Test (MDT) scheme in which a User Equipment (UE) stores its receiving performance in shadow areas, allowing a Node B to use the stored information later in optimizing service coverage.
For the LTE-A system, many discussions have been made on a CB access method to reduce the time from Service Request (SR) to resource allocation. The existing access process includes an SR trigger process, an access process, and a resource allocation process, i.e., a grant process. The term ‘grant’ refers to informing a UE of the wireless resources that the UE is granted to transmit data on an uplink. If data to be transmitted is generated in a specific UE, the UE first triggers an SR. If the SR is triggered, the UE attempts an access to send a service request to a Node B over a Physical Uplink Control Channel (PUCCH). If there is no PUCCH to be used to request a service, the UE attempts a random access process.
The random access process includes a total of 4 contention-based steps. Upon detecting a collision, a UE should re-perform the random access process from the beginning. Upon completion of the random access process, a Node B allocates resources that the UE will use. The allocated resources cannot be used by other UEs, for data transmission. In this conventional method, a UE requires many procedures and long time to be actually allocated resources.
The CB access method reduces the time required up to resource allocation by omitting the access process that is performed for exclusive resource allocation in the existing random access method. A Node B broadcasts a CB grant indicating available wireless resources. In this case, the available wireless resources can be used by any UEs in transmitting their data through contention.
Therefore, a UE determines the presence/absence of a grant broadcasted from a Node B, and if data to be transmitted is generated, the UE transmits the data using wireless resources indicated by the grant. In order for a UE to secure resources required for data transmission, this method does not include an access process and a process of assigning exclusive grants to other UEs, thereby reducing the time from SR triggering to data transmission, compared with the random access process.
The CB access method may reduce the time up to data transmission, but increases in inefficiency due to the CB disadvantageously. For example, if a plurality of UEs are assigned the same grant to transmit data, a collision occurs. In this situation, no UEs can use the associated resources. Therefore, an increase in the number of UEs desiring to use a CB grant causes a reduction in the efficiency thereof. As another problem, successive collisions may occur due to a Hybrid Automatic Repeat request (HARQ) operation. HARQ is a method in which if a receiving error for a packet occurs in a reception side, a transmission side is allowed to retransmit the packet. If a receiving error occurs, a Node B reallocates wireless resources to a UE several frames after the error occurrence so that the UE may retransmit the packet.